Notes:

This verse is, in this ground,
the 'high point of
the ghazal'. The meaning is that the longing to die that I had felt in
the night of separation-- today it has come before me, in that I have died
in the happiness of union. Other people too have used the idea of dying in
the happiness of union, but this idea is something else. And the wonders of
idiom and language, which have made the theme
of dying come alive. This verse too ought to be counted among the wonders
of the thought of Ghalib. (236)

He says, all lovers are always happy in union, but they
don't 'die of happiness' [shadii marg ho janaa]! It
seems that the longing and desire for death that I felt in the night of separation
has come before me in union. This verse is one 'lancet' [=sharp and pointed,
thus excellent, verse] among Mirza's lancets. (294)

The beloved has come, and the lover is feeling such joy
that he's about to 'die of happiness'....

The first line makes clear that the kind of happiness he's
had in union, other lovers don't have. And it also makes clear that the degree
of difficulty there was in meeting the beloved, is not there in meeting other
beloveds. And it also makes clear that the way he loved, others don't love;
and the kind of despair he felt over the chance of obtaining union, other
lovers don't feel....

From looking over the verse it can be seen that the first
line is showing the limits of happiness, and the second line is holding up
a mirror of his longing. The late Mirza Dagh
too has composed this theme,
and has composed it well:

FWP:

When we hear (under mushairah
performance conditions) the first line, the great question is, who? Somebody
masculine plural, or somebody masculine singular but receiving an honorific,
is the (colloquially omitted) subject. The speaker could also be speaking
of himself, and referring to himself colloquially as 'we'. For the content
is so abstract that we listeners really can get no idea where the verse is going.

After the usual mushairah-performance delay, when we begin
to hear the second line, we still have no idea what's going on, or who's involved,
for now we have a feminine singular verb and the vague 'night of separation',
and no means to put it all together. Only at the last possible moment are
we given the punch-word, tamannaa , followed by the
(in this case) equally necessary mire aage . Suddenly,
all at once, we (sort of) 'get' it; and in true mushairah-verse style, once it's given
us its burst of pleasure, we see that there's nothine else there, and we're
ready to move on.

But still, there remains an ambiguity in the first line that's annoying because it's poetically unrewarding. If the colloquially omitted subject is 'they', then the effect is to contrast the normal, sensible good fortune of other lovers with the uniquely bad fortune of the speaker, as described in the second line. (These others are able to be happy, they don't just suddenly collapse and die when they have attained 'union'
with the beloved.) But in that case, we have to supply the subject entirely on our own, because nothing else in the verse gives us any clue to it; the subject could also be 'you' in the third person plural, or 'he' as used for an honored person. This kind of ambiguity is really pushing the liberty of colloquial subject omission almost to the breaking point, so it feels awkward and unpleasantly lax.

Alternatively, we could imagine that the colloquially omitted subject is 'we', used by the speaker about himself. (He's scolding himself for his own undesirable, uncharacteristic behavior, before trying to explain it in the second line.) But in that case, we have to let him call himself 'we' in the first line and 'I' in the second line (because of mire aage ). This kind of shift in self-reference within a single verse almost never happens and sounds extremely awkward. Whichever choice we make, the result doesn't feel really satisfactory.

The clever use of yuu;N keeps
several interpretive possibilities open. If we take yuu;N to mean 'gratuitously,
causelessly', then it compares the normal lovers' behavior (they are happy,
but they live to tell about it) to that of the speaker, who simply collapses
and dies once he has attained union with the beloved. Or perhaps he too doesn't normally show such behavior, and it's just his own wretched fortune (his making the wrong wish during the night of separation) that has suddenly condemned him
to such a miserable, inexplicable fate.

And if we take yuu;N to mean 'like
this', then it refers to the particular kind of death that the speaker experiences:
rather than treating that death as gratuitous or causeless, the verse seeks
to explain it. In the second line, the speaker provides what seems to be a causal
explanation: his own despairing death-wish, uttered during the 'night of separation',
has now actually taken effect on him, at the most awful, unacceptable time.
The death-wish is thus potent, though uncontrollable in its timing.

Or the speaker could mean his observation in the second line simply
as an ironic meditation or commentary on the perversity of life: during the
night of separation he longed for death in vain, and now that it's fantastically
unwanted and inopportune, sure enough here it is before him. Death comes ineluctably
in its own time; as we're reminded in {191,7}. (For more on yuu;N , see {30,1}.)

Another possible interpretation is that of the commentators:
that the lover is specifically 'dying of happiness', for which Urdu has the
excellent idiom shadii marg ho janaa . That sounds like
a plausible extrapolation, but there's no particular warrant for it in the
verse. As Bekhud Mohani observes, on this interpretation the contrast with
normal lovers serves to highlight the unique power of the true lover's passion:
his extreme despair in separation, and his extreme bliss in union, two unendurable
intensities that seem to cause him to drop dead from the contrast between
them, or from the sheer intensity of his joy. Which is all very well, but
it's not what interests the speaker when he gives us his own reflections on
the subject.

For another verse in which 'normal' behavior is contrasted
with that of one particular lover, see {111,5}.